There are systems where a radio transmits a radio frame while changing the radio frame length (time length) or amplitude to communicate the meaning of the data of the payload of the radio frame and, in addition, a receiver that has received the radio frame detects the changes in frame length or amplitude of the radio frame and treats the detected changes in frame length or amplitude as information such that, when the transmitter transmits a radio signal corresponding to an identifier and the receiver receives the identifier, the transmitter can remote-control the receiver, for example, issue an instruction to transition from sleep mode to active mode upon reception of that identifier.
If such a system uses for its radio scheme a wireless local area network (LAN) compliant with IEEE 802.11 in the Industry Science Medical (ISM) band, the transmitting radio includes a synchronous detection circuit and performs carrier sensing using Carrier Sense Multiple Access with Collision Detection (CSMA/CA) based on the strength of signals that have been band-limited using the frequency span of one particular wireless channel (i.e. the width of 22 MHz) out of separate channels CH1 to CH 14 in the ISM band and, based on the results, transmits radio frames.
In the case of typical wireless LAN communication, a radio that receives radio frames uses a radio interface capable of synchronous detection to demodulate and decode the wireless LAN frames to payload data.
In implementations where changes in frame length or amplitude are detected, signals that have been band-limited by an RF filter are received using envelope detection, which is a type of asynchronous detection, in order to simplify its circuitry and process and to reduce power consumption.
In the case of synchronous detection, signals that have been band-limited by an RF filter are converted to intermediate frequency signals or baseband signals, and a narrower band filter is applied to the frequency after conversion for band-limiting to a desired bandwidth. In the case of asynchronous detection, frequency conversion does not occur; accordingly, signals in a frequency bandwidth adapted to the characteristics of the RF filter are observed.
Most commercial RF filters that can pass the ISM band of 2.4 GHz have relatively broad bands so as to cover the entire ISM band (channels CH1 to Ch14 (i.e. the width of about 100 MHz)). Thus, it is not realistic for reasons of cost and mounting area to add a narrow band filter.
Conventionally, in wireless communication schemes with asynchronous detection, it is known to use the presence and absence of radio frames or changes in the amplitude of radio frames as control identifiers (Patent Document 1).
Also, it is known to control the timing for transmitting radio frames using IEEE 802.11 compatible signals and, depending on the presence or absence of a radio frame, transmit an On OFF Keying (OOK) signal (Patent Document 2).
Patent Document 1: JP 2009-055533 A
Patent Document 2: JP 2009-077375 A
Non-Patent Document 1: E. H. Armstrong, “Some recent developments of regenerative circuits”, Proc. Inst. Radio Eng., Vol. 10, pp. 244-260, Aug. 1922.